1. Field of the Invention
The present invention relates to a process for producing hydroanthraquinones obtained by Diels-Alder reaction of naphthoquinone with 1,3-butadiene especially hydroanthraquinones including 1,4,4a,9a-tetrahydroanthraquinones.
2. Description of the Prior Art
The hydroanthraquinones including 1,4,4a,9a-tetrahydroanthraquinone (hereinafter referring to as THAQ) and 1,4-dihydroanthraquinone (hereinafter referring to as DHAQ) are important as starting materials for producing anthraquinone derivatives such as 1-aminoanthraquinone and 2-aminoanthraquinone and also anthrones such as benzanthrone. Recently, the hydroanthraquines have been considered to be important as a digesting assistant for alkaline pulp.
THAQs have been usually produced by Diels-Alder reaction of 1,3-butadiene (hereinafter referring to as BD) with naphthoquinone (1,4-naphthoquinone otherwise specified) obtained by an oxidation of naphthalene. This reaction has been usually carried out in an organic solvent.
In order to separate THAQs from the reaction mixture, it is convenient to separate crystals of THAQs precipitated by cooling the reaction mixture, by a filtration etc. It has been found that when the reaction mixture obtained by Diels-Alder reaction is cooled to crystallize the product and the resulting slurry is filtered, the filtration is remarkably difficult and the resulting hydroanthraquinones contain impurities other than derivatives having anthraquinone ring to cause lower purity.